linux-kernel - Re: [PATCH] locking/mutex: Test for initialized mutex

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Date:   Wed, 10 Jul 2019 17:50:54 +0100
From:   Will Deacon <will@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, tglx@...utronix.de
Subject: Re: [PATCH] locking/mutex: Test for initialized mutex

On Wed, Jul 03, 2019 at 11:21:26AM +0200, Sebastian Andrzej Siewior wrote:
> An uninitialized/ zeroed mutex will go unnoticed because there is no
> check for it. There is a magic check in the unlock's slowpath path which
> might go unnoticed if the unlock happens in the fastpath.
> 
> Add a ->magic check early in the mutex_lock() and mutex_trylock() path.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> Nothing screamed during uninitialized usage of init_mm's context->lock
>   https://git.kernel.org/tip/32232b350d7cd93cdc65fe5a453e6a40b539e9f9
> 
>  kernel/locking/mutex.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 0c601ae072b3f..fb1f6f1e1cc61 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -908,6 +908,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  
>  	might_sleep();
>  
> +#ifdef CONFIG_DEBUG_MUTEXES
> +	DEBUG_LOCKS_WARN_ON(lock->magic != lock);
> +#endif

Why do we need to check this so early, or could we move it into
debug_mutex_lock_common() instead?

Will